The present invention relates to a liquid extracting apparatus suitable for an application to a filtrate tank and an absorber tank of a wet type flue gas desulfurization system, for example.
In recent years, as a flue gas desulfurization technique for mainly removing sulfur oxides such as sulfur dioxide from the flue gas produced in a thermal power plant etc., a wet type limestone-gypsum process of tank oxidation system has been used widely in which absorbent slurry in which calcium compounds such as limestone is suspended is sent from an absorber tank at the bottom of an absorber to a gas-liquid contact section at the upper part of the absorber to be brought into air-liquid contact with flue gas while being circulated, and oxidizing air is blown compulsorily into the absorber tank to oxidize the slurry having been brought into air-liquid contact in the absorber tank, by which gypsum is produced as a by-product.
In this flue gas desulfurization system, some of the slurry in the absorber tank is extracted to effect solid-liquid separation in order to recover gypsum. A filtrate produced by this solid-liquid separation is generally sent once to a filtrate tank, and thereafter some is returned to the absorber tank, and the remaining is treated as desulfurized waste water. This waste water treatment is performed to prevent harmful impurities such as chlorine in the flue gas from accumulating while being dissolved in the liquid content circulating in the flue gas desulfurization system. Depending on the capacity etc. of a waste water treatment system, a certain limitation is sometimes imposed on the suspended solid concentration (so-called SS concentration) of the waste water.
According to the performance of the solid-liquid separator for effecting the solid-liquid separation, the suspended solid concentration of the filtrate sometimes exceeds this limit. In this case, it is necessary to extract a liquid content with a relatively low suspended solid concentration from the filtrate in the filtrate tank and to send the same to a waste water treatment system as the desulfurized waste water.
Also, in the above-described flue gas desulfurization system, in order to achieve stable operation, it is necessary to regulate the slurry extraction amount etc. for recovering gypsum to keep the amount of slurry (liquid level) in the absorber tank in a certain range, and on the other hand it is also necessary to keep the slurry concentration (suspended solid concentration mainly consisting of gypsum) in the absorber tank in a certain range (normally, 20 to 30%).
The reason for this is as follows: If the slurry concentration is too high, a trouble such as clogging of a pipe or a pump in a circulation system for sending slurry to the upper part of the absorber and in a line for extracting slurry from the absorber tank is liable to occur, leading to difficulty in operation. If the slurry concentration is too low, so-called gypsum seed crystal in the slurry decreases, and most of the gypsum yielded successively by a reaction in the absorber tank caused by the absorption of sulfur dioxide deposits in a state such as to adhere to the surface of the equipment component, for example, to the inside wall surface of the absorber tank, which gypsum turns to scale to cause a trouble such as clogging of a pipe. Also, if the slurry concentration is low, a load for the treatment in which the extracted slurry is separated into solid and liquid to recover gypsum increases, which is disadvantageous in terms of operation cost.
The amount of sulfur dioxide in the flue gas always varies depending on the power generation load, etc. Therefore, the supply amount of absorbent (limestone, etc.) charged into the absorber tank must always be regulated to the necessary minimum amount according to the inlet sulfur dioxide amount thus varying. Also, a fixed amount of cleaning water for an air sparger or the like for blowing oxidizing air always flows into the absorber tank.
Therefore, for example, in a state in which the amount of sulfur dioxide in the flue gas is small (low-load state), the amount of solid content charged into the tank or produced in the tank is small. On the other hand, since a fixed amount of the aforementioned cleaning water always flows in, the concentration of slurry in the tank tends to decrease. In particular, when a wet cooling type dust removing section is provided on the upstream side of the absorber (in the case of soot separation system), the flue gas flowing into the absorber has almost been saturated by water vapor, so that the amount of water which evaporates in the absorber and is brought away by the flue gas is very small, and therefore, there is a high possibility of the concentration of slurry in the tank decreasing beyond the aforementioned certain range during the operation.
Even when the system is stopped (desulfurization is stopped), the cleaning water is usually caused to flow continuously. In this case, therefore, even if the soot separation system is not used, the concentration of slurry in the tank decreases beyond the aforementioned certain range.
Thereupon, in such a case, it is necessary to positively extract only the liquid content with a low suspended solid concentration from the absorber tank.
Conventionally, however, there is no simple and effective means for extracting the liquid content with a low suspended solid concentration from the filtrate tank or the absorber tank.
As a conventionally known apparatus of this type, a liquid extracting apparatus (an extractor for extracting clean liquid from suspension) has been disclosed in Japanese Utility Model Provisional Publication No. 60-159350 (No. 159350/1985). This apparatus is configured so that a cylinder whose lower end only is open and which is provided with a grating in the lower end opening is arranged so as to be submerged in suspension (slurry), and a suction pipe for a pump is connected to the closed upper end of the cylinder. The pump is operated to extract the liquid at a slow speed (for example, 1 m/h to 10 m/h) not higher than a predetermined flow velocity via the cylinder, by which the precipitation and separation of solid content in the cylinder are caused, thereby removing the liquid content with a low suspended solid concentration.
However, the liquid extracting apparatus disclosed in the above-described Publication has the following problems to be solved.
When the flow rate of the liquid content to be extracted is high or when the target suspended solid concentration in the liquid content to be extracted is low, in order to satisfy the condition at which the extraction speed is not higher than the predetermined flow velocity, the inside diameter of the cylinder must be increased, or a large number of cylinders must be provided. In this case, it is difficult to effectively extract only the liquid content with a low suspended solid concentration, or the construction of apparatus is complicated and expensive.
Specifically, if the inside diameter of the cylinder is merely increased, the aspect ratio (the ratio of inside diameter to length) of the cylinder increases inevitably, so that the influence of movement of the liquid in the tank (outside the cylinder) is liable to spread to the inner part of the cylinder. Therefore, the turbulence of flow of the liquid occurs excessively in the cylinder, so that even if the aforementioned condition is satisfied in terms of the average flow velocity, a portion where the upward flow is fast is produced partially, whereby the precipitation and separation are not attained effectively. As a result, the liquid content with a sufficiently low suspended solid concentration cannot be extracted.
Also, if an attempt is made to accommodate a high flow rate by merely providing a large number of cylinders, an increase in cost for installing a number of cylinders poses a problem. In particular, since the upper end of cylinder is closed, the manufacturing cost of the cylinder itself is high, so that if a number of cylinders are disposed in a state so as to be submerged in the slurry, a significant increase in cost occurs including the cost for installing members for supporting the cylinders.
The above-described Publication has disclosed a technique in which a grating is provided in the lower end opening of the cylinder to lessen the influence of liquid movement.
However, the rough grating with a high aspect ratio (the ratio of width to length) of each section as shown in FIG. 2 of the above-described Publication provides insufficient performance to lessen the influence of liquid movement. In particular, in the case where the inside diameter of cylinder must be increased as described above because of a high flow rate of liquid content to be extracted or in the case of a tank in which an agitator is provided to cause the slurry in the tank to move heavily, such as the aforementioned absorber tank and filtrate tank of flue gas desulfurization system, the flow of liquid in the cylinder is liable to be made turbulent by the influence of liquid movement outside the cylinder.
If an attempt is made to merely decrease the width of each section of the grating in order to enhance the performance of the grating, the manufacturing cost of cylinder further increases, which likewise suffers a disadvantage in terms of cost. The reason for this is as follows: As shown in FIG. 2 of the above-described Publication, the structure of the grating is thought to be such that, for example, the side edges of a plurality of partition plates are fixed to the inner peripheral surface on the lower end side of the cylinder, so that the manufacture of cylinder is originally troublesome. If an attempt is made to form this structure in large numbers with a fine mesh provided in a large cylinder, a considerable increase in cost occurs. Although the specific construction in which the grating is formed and assembled to the cylinder has not been described at all in the above-described Publication, it is apparent that when, for example, an attempt is made to accommodate a high flow rate, a significant increase in cost occurs depending on the mode, inferring from the structure shown in FIG. 2 of the above-described Publication.
Accordingly, an object of the present invention is to provide a simple and less-costly liquid extracting apparatus that can effectively extract only liquid content in slurry in a tank even if the slurry in the tank moves.
To achieve the above object, the liquid extracting apparatus of a first embodiment of the present invention is a liquid extracting apparatus for discharging slurry liquid content with a lower suspended solid concentration than that of slurry in which solid content is suspended in liquid from a tank in which the slurry is stored, comprising:
a liquid content introduction path configured so that the lower end thereof is open toward the bottom surface of the tank so that the slurry can be introduced from the lower end side to achieve the precipitation and separation of solid content; liquid extracting means connected to the upper end side of the liquid content introduction path to extract slurry liquid content from the tank via the liquid content introduction path; and flow straightening means provided at the lower end portion of the liquid content introduction path. The flow straightening means is preferably configured so that the lower end portion of the liquid content introduction path is divided into a number of long flow paths along the flow direction.
Also, the liquid extracting apparatus of a second embodiment is characterized in that the flow straightening means is configured so that a number of tubes are provided at the lower end portion of the liquid content introduction path in a packed state.
Also, the liquid extracting apparatus of a third embodiment is characterized in that the flow straightening means is configured so that a number of tubes are placed closely on a net-shaped member installed so as to cover the lower end opening of the liquid content introduction path.
Also, the liquid extracting apparatus of a fourth embodiment is characterized in that the flow straightening means is configured so that a plurality of plate-shaped members in which a number of chambers or holes that are open on the upper and lower face sides are formed over the whole face thereof are stacked as necessary, and are disposed so as to occupy the whole of lower end portion of the liquid content introduction path; and a number of long flow paths are formed along the flow direction by the chambers or holes.
Also, the liquid extracting apparatus of a fifth embodiment is characterized in that the liquid content introduction path is formed by partitioning a part of the tank by a partition wall whose upper edge projects above the liquid level in the tank and whose lower edge extends below the liquid level in the tank, and the upper end side of the liquid content introduction path is also open.
Also, the liquid extracting apparatus of a sixth embodiment is characterized in that the liquid extracting means is formed by an outflow port of slurry formed at a position lower than the slurry liquid level on the side wall of the tank, so that slurry in the tank is caused to flow out by a head difference.
Also, the liquid extracting apparatus of a seventh embodiment is characterized in that the liquid extracting means is connected to a plurality of locations in the radial direction of the liquid content introduction path.
Also, the liquid extracting apparatus of an eighth embodiment is characterized in that the interior of the liquid content introduction path is divided by walls for each location to which the liquid extracting means is connected.
According to the above-described liquid extracting apparatus in accordance with the present invention, only the liquid content can be extracted effectively from the tank in a manner described below by using a simple and less-costly construction in which the liquid extracting means is connected to the liquid content introduction path provided with flow straightening means.
Since the flow straightening means has a construction such that the lower end portion of the liquid content introduction path is divided into a number of long flow paths along, the flow direction, the influence of slurry movement in the tank is lessened sufficiently and does not reach the interior of the liquid content introduction path, regardless of the inside dimensions of the whole of the liquid content introduction path. Therefore, if the inside dimensions of the liquid content introduction path (cross-sectional area of flow path) are set so that the average flow velocity in the liquid content introduction path is lower than the precipitation speed of solid content, the precipitation and separation of solid content in the liquid content introduction path are attained effectively. As a result, even when a high flow rate is extracted, or when a liquid content with a lower suspended solid concentration is extracted (that is, when the inside dimensions of the liquid content introduction path cannot be set large), a predetermined amount of only the liquid content with a concentration not higher than the predetermined value can be extracted with high reliability.
As in the case of the above-described second embodiment, when the flow straightening means is configured by providing a number of tubes at the lower end portion of the liquid content introduction path in a packed state, the manufacturing cost and assembling cost of the flow straightening means are reduced, which is particularly advantageous when a liquid content introduction path with large inside dimensions is used.
In particular, as in the case of the above-described third embodiment, when the flow straightening means is configured by placing a number of tubes closely on the net-shaped member installed so as to cover the lower end opening of the liquid content introduction path, the assembling of the flow straightening means is completed by a very simple operation such that, for example, a number of tubes are dropped onto the net-shaped member after the net-shaped member is installed.
Also, as in the case of the above-described fourth embodiment, even when the flow straightening means is configured by stacking, as necessary, the plurality of plate-shaped members in which a number of chambers or holes that are open on the upper and lower face sides are formed over the whole face thereof and by disposing the plate-shaped members so as to occupy the whole of lower end portion of the liquid content introduction path, the liquid content can be extracted effectively at a low cost.
In this case as well, since the chambers or holes are stacked as necessary and communicate with each other, whereby a number of long flow paths are formed along the flow direction, the influence of slurry movement in the tank can be lessened sufficiently. Also, since a structure such as the plate-shaped member described above can be manufactured easily, for example, by integral molding of a synthetic resin, the installation cost is relatively low even when the apparatus is large in size.
Also, as in the case of the above-described fifth embodiment, when the liquid content introduction path is formed by partitioning a part of the tank by a partition wall whose upper edge projects above the liquid level in the tank and whose lower edge extends below the liquid level in the tank, and the upper end side of the liquid content introduction path is also open, even if air bubbles flow into the liquid content introduction path, most of the air bubbles escape from the upper end opening of the liquid content introduction path and is not mixed in the extracted liquid. Therefore, a trouble is avoided such that smooth extraction of liquid content is hindered by the air bubbles or a pump constituting the liquid extracting means is damaged.
Further, in this case, the cost for installing the liquid content introduction path can be reduced as compared with the case where the upper end of the liquid content introduction path is closed.
Also, as in the case of the above-described sixth embodiment, when the liquid extracting means is formed by an outflow port of slurry formed at a position lower than the slurry liquid level on the side wall of the tank, and therefore slurry in the tank is caused to flow out by a head difference, the liquid content can be extracted without the use of a pump, so that the pump installation cost and the pump power can be saved.
Also, as in the case of the above-described seventh embodiment, when the liquid extracting means is connected to a plurality of locations in the radial direction of the liquid content introduction path, the inflow of liquid into the liquid extracting means is effected at the plurality of locations in the radial direction of the liquid content introduction path. Therefore, for example, when a large amount of liquid content is to be extracted by increasing the inside dimensions of the liquid content introduction path, the deflection of flow velocity caused by the inflow of liquid from the liquid content introduction path to the liquid extracting means effected concentrically at a part in the radial direction of the liquid content introduction path can be reduced.
Also, as in the case of the above-described eighth embodiment, when the interior of the liquid content introduction path is divided by walls for each location to which the liquid extracting means is connected,. convection caused by temperature difference etc. in the liquid content introduction path is restrained, so that the liquid content of a high flow rate can be extracted reliably as a whole. Specifically, for example, when a large amount of liquid content is to be extracted by merely increasing the inside dimensions of the liquid content introduction path, even if the influence of slurry movement outside the liquid content introduction path can be lessened by the flow straightening means, there is a fear of convection caused by temperature difference etc. in the interior of the liquid content introduction path. In the case of the present invention, however, the liquid content introduction path is divided into narrow introduction paths, and the liquid extracting means (for example, a pump) is connected for each of the sections, so that such convection is less liable to occur. Therefore, the reliability is further enhanced when the liquid content of a high flow rate is to be extracted, or when the liquid content with a lower suspended solid concentration is to be extracted.